Acoustic resonances are a familiar problem in the operation of high-pressure gas discharge lamps. Depending on the geometry and on the pressure in the lamp, these resonances occur in a frequency range between 5 kHz and 1000 kHz and can lead to arc irregularity and even to the destruction of the lamp in the case of distinct resonances. Operating a lamp with an alternating current which has a frequency in said frequency range is therefore not absolutely reliable.
For this reason, operating devices which operate the lamp in so-called squarewave mode have become widely used on the market. However, the squarewave mode requires great circuit complexity which is why there are efforts to operate the lamp in so-called high frequency mode in spite of the risk of acoustic resonances. In this mode, the lamps are supplied with an alternating current in the specified
frequency range because an operating device can be implemented particularly cost-effectively especially in this frequency range.
In the document US 2003/0111968A1 (Trestman), an operating device is described which operates a lamp at an operating frequency which is frequency modulated. In this arrangement, a frequency range is selected in which the lamp does not have any distinct acoustic resonances. So that these weak resonances are not excited, the operating frequency is continuously varied around a center frequency within a range of 50 kHz. The modulation is controlled by a residual ripple of a supply voltage. The document specified talks about a constant supply voltage which has an actually unwanted residual ripple of, for example, 6 Vrms which is caused by a feeding line voltage. Due to the rectification, the residual ripple has 120 Hz at a line frequency of 60 Hz. The lamp current thus has an operating frequency which is modulated by +/−50 kHz with a modulation frequency of 120 Hz.
The disadvantageous factor in the prior art described is that a frequency range must be found in which the lamp only has weak resonances. The operating frequency range covered by the modulation thus avoids frequency ranges in which strong major resonances of the lamp to be operated occur. The result in the prior art is that a frequency range in which the operating frequency occurs must be adapted to the lamp to be operated. The prior art does not guarantee that two lamps which have comparable performance data can be operated on the same operating device.